Conventional liquid level sensors for fixed, discrete and continuous level sensing are not effective to measure fluid levels in highly aerated fluids. For example, ultrasonic sensors, which determine the presence or level of a fluid by the passage of acoustic waves from a transmitting element to a receiving element (Lynnworth, Physical Acoustics, Academic Press, 1979, pgs. 460-461), do not function in highly aerated fluids due transmission losses that drastically increase with aeration. Further, sensors that determine fluid level using a buoyant element can operate in aerated fluids, but have moving parts subject to jamming and wear.
One potential solution to this aeration problem is the use of acoustic modes with significant out of plane displacements. Rayleigh surface acoustic waves, for example, suffer large propagation losses in the presence of aerated fluids on the immersed substrate surface. A problem with using Rayleigh surface waves lies with fluid remaining on non-immersed surfaces of a sensor probe causing additional propagation losses because the sensor cannot discriminate between fluid residue on the probe, and the fluid in which the probe is immersed. This can occur, for example, in highly viscous fluids at low temperatures, because the fluids are slow to drain from a recently immersed surface.
A further problem with out of plane, or longitudinal modes is related to the need to seal the probes. This is normally accomplished with compliant elastomeric polymers in the form of an O-ring. Out of plane modes are greatly absorbed by these polymer O-rings, which are tightly compressed to affect a seal over a wide temperature range, and with a possible pressure differential between the inner and outer regions. Accordingly, a need exists for a liquid level sensor capable of measuring the level of aerated liquids, without the use of moving parts, that is less susceptible to sealing losses and non-immersed fluid residue.